In obstetrics as well as in research a pressure meter catheter is introduced through the cervix into the uterus for close labor monitoring. The instrument detects any pressure variation occurring as to its change and relative magnitude (with respect to the ambient pressure outside of the uterus) and converts it into signals which are applied to a visual display unit for display and, if desired, to a recording apparatus for registration.
As with any sensor use, calibration is a fundamental requirement to be met prior to putting the pressure meter catheter to practical use so as to compensate, at least at a test point, the internal mistakes inherent in the system and possibly also external ones caused by marginal conditions.
In principle, this calibration could be made at any suitable pressure. However, as a rule, ambient pressure (atmospheric pressure) is preferred at which the relative pressure to be measured is zero. In this context, therefore, we do not speak simply of calibration but instead of zero setting. To accomplish that, the ambient pressure is taken as the reference or standard pressure, and the difference, indicated or recorded, from the measured pressure taken by the pressure meter catheter is adjusted to zero by manipulating the display or recording means.
There are two types of catheters for measuring pressure: With the first one, the pressure prevailing in the uterus is transmitted by a liquid column carried in a tube from a proximal measuring point which is isolated by a membrane to a distal pressure sensor which represents the signal transmitter of the measuring arrangement.
If designed properly, the pressure sensor can be disengaged readily from the pressure meter catheter even when the latter is in situ so that the zero setting of the sensor can be effected any time. Apart from a number of disadvantages which are less relevant in the present context, such a pressure meter catheter has the important disadvantages that bubbles in the pressure transmitting liquid have an attenuating and, therefore, falsifying effect and that the hydrostatic measuring error resulting from the vertical difference between the proximal measuring tip and the pressure sensor can be estimated only roughly because the exact location of the measuring tip either is unknown or changes as the patient's body moves.
Therefore, a pressure meter catheter of a second type was proposed to overcome those disadvantages. With this type, the pressure sensor is not located at the distal end of the catheter tube but instead at the proximal end thereof, exactly where the measuring location is. A pressure meter catheter of this kind is sold, for instance, under the name of "INTRAN" by the Utah Medical Products, Inc.
It may be gathered from the instructions for use published by the company for their pressure meter catheter that the zero setting is realized before introducing the catheter as long as its measuring tip is exposed to ambient atmospheric pressure. Furthermore, it is explained in the instructions for use that renewed zero setting is not required once the measuring tip is in situ.
That, however, is true only as long as there are no disturbances. Yet if the displaying or recording unit should fail due to an operating fault or equipment failure during a measurement which may last several hours (e.g. for reasons of inadvertent disconnection of a mains cable when connecting another instrument) then a new zero setting is indispensable. To do that, the measuring tip of the catheter would have to be exposed once more to the ambient air in order to be subjected to atmospheric pressure. Of course, following such a procedure, the measuring tip no longer would be sterile. For this reason another, sterile pressure meter catheter is subjected to zero setting under sterile conditions and is introduced into the cervix upon removal of the former catheter. It need not be explained in detail that any such procedure is cumbersome and expensive, all the more so as it may prove to be necessary when the course of the parturition being monitored takes a rapid and often dramatic turn so that any manipulating of a catheter may be greatly disturbing.
It is another disadvantage of the known catheter that the sterile packing enveloping the catheter must be opened for zero setting so that the catheter is exposed at least in part during the zero setting and, therefore, subject to contamination which gives rise to another problem of sterility.
A pressure meter catheter of the kind mentioned is applied not only in human medicine and obstetrics but also wherever the occurrence of considerable pressures in body cavities of human beings and animals permitting access of pressure sensors is to be monitored, examples being the cud of a ruminant or blood streams.